1. Field of the Invention
This invention relates generally to modulating signals, and more particularly, to a method and apparatus for modulating a signal using gaussian filtered minimum shift keying with a flexible sampling rate.
2. Description of the Related Art
The proliferation of digital devices has increased demand for wireless transmission of binary or digital data. Binary data includes sharp transitions between logic levels, such as from zero to one and one to zero, resulting in a signal having harmonic content requiring considerable bandwidth. One method for reducing the bandwidth requirement is pre-modulation filtering, which involves translating the data bits into a particular combination of phase, frequency, or amplitude prior to modulating an RF carrier with the digital data. One such pre-modulation technique is gaussian filtered minimum shift keying (GMSK), where a gaussian filter is used to reduce the harmonic content of the modulated signal, thus reducing the required bandwidth.
Previous GMSK techniques are inflexible and require complex hardware. Changes in parameters, such as the sampling frequency of the transmitted signal, require wide scale hardware changes. For example, consider a system receiving data for transmission at a particular receive sampling rate. The interval between digital data bits is referred to as the bit interval. The system must generate a number of samples per bit interval to generate the baseband signal. The rate at which samples are generated for the transmitted data is referred to as the transmit sampling rate. The digital signal processor must supply data at the transmit sampling rate. If the desired transmit sampling rate is changed, the digital signal processor must be reconfigured to supply data at the new desired transmit sampling frequency. This increases the demand on the digital signal processor, and at high transmit sampling rates, may decrease its performance. Such an arrangement is inflexible and costly to adapt to varied operating conditions.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.